I. Field of the Invention
The present invention relates to a process for preparing a microcellular polyurethane foam with improved green strength. More particularly, the present invention relates to a process for preparing flexible polyurethane foams having improved green strength.
II. Description of the Prior Art
High density microcellular polyurethane foams are used by the automotive industry to prepare such molded parts as, e.g., fascia, air dams, fender flares, spoilers, and fender extensions. Many of these parts are quite complex in configuration; and molding of such parts can be difficult when the parts have wrap-arounds or undercuts. Problems occur on demold if the parts tear as they are pulled or ejected off of mold cores or cavities.
The propensity of a molded polyurethane part to show surface cracking if bent over sharply soon after demold is an indication of its "green strength." Ordinarily, the production suitability of a particular flexible microcellular polyurethane foam otherwise meeting the specific physical property requirements for a given application may be judged on the basis of its "green strength," i.e., the resistance to mechanical damage such as, tearing, non-recoverable stretching, or cracking at the time of demold. The degree of green strength required may be dependent on the complexity of the part produced. It is apparent that a simple flat part would not require the degree of green strength necessary for a complex shape with numerous cut-out sections, curves and undercuts. Hence, conversely, the poorer the green strength of a polyurethane foam, the more likely it is that said foam will be unsuitable for producing complex, molded parts.
The prior art has not provided a totally satisfactory solution to providing high density microcellular flexible foams with good green strength. Some of the relevant prior art of which the applicant is aware includes U.S. Pat. Nos. 3,580,868; 3,620,986; 3,894,912 and 3,922,238.
U.S. Pat. No. 3,580,868 teaches that catalysts prepared by reacting dimethylamine, formaldehyde, and phenols may be used to catalyze the polymerization of compounds which contain one or more isocyanate groups in the molecule. The patentees disclosed that the prior art " . . . amine components used cause the isocyanate polymerization to proceed insufficiently smoothly so that the product obtained is commercially unsatisfactory in every respect." (column 1, lines 60-64).
U.S. Pat. No. 3,620,986 discloses certain mononuclear Mannich bases of secondary amines, formaldehyde and phenols; it also discloses a process for the production of synthetic resins containing isocyanurate groups which comprises polymerizing an organic polyisocyanate in the presence of these Mannich bases. The patentees disclose that "the use of alkoxylated condensation products of amines . . . in the reaction of isocyanates . . . for the production of foams, is also known, although in these cases no substantial polymerisation of the isocyanate groups can be observed." (Column 1, lines 39-44).
U.S. Pat. No. 3,894,972 discloses a process for preparing rigid cellular foam compositions by condensing an organic polyisocyanate in the presence of a furfuryl alchohol and a tertiary amine.
U.S. Pat. No. 3,922,238 discloses a process for preparing rigid cellular foam compositions by condensing an organic polyisocyanate in the presence of a blowing agent, a polyol, and a catalyst system containing an alcohol, a tertiary amine trimerization catalyst, and a urethane catalyst.
Other art which may be deemed of interest hereto includes U.S. Pat. Nos. 3,050,477; 3,112,281; and 3,471,416. U.S. Pat. No. 3,050,477 teaches a polyurethane foam having uniform cell size which is achieved through the use of organosilane surfactants. The silanes may be used alone or in admixture with a diol modifier. The foams of the reference may be flexible, rigid or semi-rigid depending on the linearity or branching of the isocyanate and/or polyether reactant as well as the blowing agent. The blowing agents disclosed by the reference include the well-known halohydrocarbons as well as water or mixtures thereof. The reference further teaches the use of conventional tertiary amine catalysts as well as other organometallic catalysts. The catalyst is employed in an amount ranging from about 0.05 parts to about 3.0 parts by weight per hundred parts by weight of polyol.
U.S. Pat. No. 3,112,281 teaches the use of a tetra (polyoxypropylene) ethylene diamine as a polyol reactant in the preparation of polyurethane foams. The reference teaches that when using this polyol to manufacture flexible foam, it is employed along with silicone oils, water as a blowing agent, and a conventional tertiary amine catalyst.
U.S. Pat. No. 3,471,416 teaches a rigid polyurethane foam using a combination of polyols, including tetra (hydroxyalkyl) alkylene diamine and polyhydric alcohols, an organic polyisocyanate, a wetting agent, a blowing agent, and a specific amine catalyst mixture.